Methane concentrations increase in bottom waters during summertime anoxia in the highly eutrophic estuary, Chesapeake Bay, U.S.A.

Abstract

The Chesapeake Bay is a eutrophic estuary that undergoes seasonal bottom water hypoxia. Methane (CH4) gas bubbles are found within the upper sediment layers, but whether this greenhouse gas escapes to the water column or the atmosphere is not well known. Here, we hypothesize that when bottom waters become anoxic, CH4 is released from the sediments, builds up under the pycnocline, and escapes to the atmosphere at the end of hypoxia. Osmotically powered pumps (OsmoSamplers) were used for the first time to test this hypothesis. They were deployed from April to October 2013 and continuously collected bottom water into small diameter, copper tubing. Upon recovery, the tubing was cut into 4 d increments, and the enclosed water extracted and measured for CH4 concentrations. Results showed that in April, CH4 concentrations were low (∼1 μM) when bottom waters were fully oxygenated and increased as anoxic conditions set in. By mid-July, CH4 concentrations peaked and reached as high as 40 μM, most likely coming from the sediments. By early August, concentrations decreased until they returned to background levels when normal oxygen conditions returned in late September. While most of the built-up CH4 was gone by the end of anoxia, CH4 concentrations measured in discrete surface water samples in June and September suggest that there was still a significant flux of CH4 to the atmosphere. Taken together, our time-series data shows that the CH4 flux from the Chesapeake Bay is temporally variable, potentially significant, and dependent on physical processes (e.g., storms) occurring in the Bay.